1. Field of the Invention
The present invention relates in general to a photoconductive film adapted for use in a target of a photoconductive image pick-up tube. More particularly, the invention relates to an improved structure of the photoconductive film of a rectifying contact type having an enhanced sensitivity for red light which is substantially free from drawbacks such as variations in light sensitivity and after-image caused by a long and continuous use of the image pick-up tube incorporating therein such photoconductive film.
2. Description of the Prior Art
It is well known that amorphous selenium exhibits photoconductivity, has a p-type conductivity, and is capable of forming a rectifying contact when contacted with a material having an n-type conductivity. By making use of such properties, a photoconductive target film of the rectifying contact type can be advantageously manufactured from amorphous selenium. However, selenium has a drawback that its sensitivity for long wavelengths is inherently very low. With a view to eliminating such drawback, it is known to add tellurium (Te) to the photoconductive film of selenium (Se). However, the addition of Te to the Se film will often cause a reduction in the carrier mobility and impair the characteristic desired for the target of the image pick-up tube. In order to overcome such undesirable situation, it has already been proposed to add Te only to a part of the photoconductive film as is taught in U.S. Pat. No. 3,890,525.
The target of the image pick-up tube disclosed in U.S. Pat. No. 3,890,525 is composed of a transparent base plate, a transparent electrode made of thin oxide, indium oxide, titanium oxide or the like or a mixture thereof and a photoconductive film of Se, As and Te, wherein the rectifying contact is formed between the transparent electrode and the photoconductive film. In the photoconductive film, As is distributed uniformly along the direction of the thickness of film with a concentration of 10 atomic %. On the contrary, the concentration distribution of Te is nonuniform in such a manner tht Te is dispersed with a concentration lower than 10 atomic % in the interface region between the photoconductive film and the transparent electrode and, as the distance from the transparent electrode increases, the concentration of Te is progressively increased to attain a maximum value of 10 to 40 atomic % and again decreased to 10 atomic %. The maximum concentration of Te is located between the transparent electrode and the middle portion of the photoconductive film. The distribution of Te in such pattern does not impair the property of Se but provides an advantage that the sensitivity of the target to red light is enhanced.
However, it has been found that, with the above described structure of the target, signal current will undergo undesirable variation when the target has been operated continuously for a long time, illuminated by a long wavelength light, or a so-called after-image is stressed when one and the same object has been continuously picked up.